Assuring the safety of fluid milk, related dairy products, and juices while maintaining quality and increasing the shelf life of products is a significant challenge for the food industry. Many perishable organic liquids, including juices, but especially raw milk, serve as suitable growth mediums for microorganisms. Benefits in distribution and organic liquid quality could be derived from reducing microbial growth.
The bulk transport of perishable organic liquids generally requires at least one of the following: pasteurization or similar treatments to reduce, eliminate or control pathogens; rapid shipment; and in some cases, refrigeration. Each of these options imposes additional cost and or limitations. For instance, shipment by truck may be the quickest transport time but still may not be sufficiently rapid to reach all markets. Shipment by rail or ocean cargo vessel is slower but more economical. Refrigerated shipping costs are substantially higher than the cost for shipments not requiring refrigeration. Furthermore, refrigeration is not effective to adequately restrain the growth of psychrotrophic microorganisms capable of activity at temperatures below 7° C. over sustained intervals of time. Each process of pasteurization or similar pathogen reduction treatment imposes not only expense, but may also negatively impact the flavor quality, nutritional content, and other sensory characteristics, such as color, of the treated organic liquid with a resulting negative market impact. Additionally, thermoduric microorganisms that are potential pathogens or cause spoilage may survive the pasteurization process.
As a result of these concerns, today when arranging for the shipment of fresh milk from the continental United States to Hawaii or a Caribbean island without significant dairy herds, there are two principal options, namely:    Milk is pasteurized before bulk shipment and is re-pasteurized prior to local packaging for retail sale. The result is a flavor not as fresh as with single pasteurization and a higher cost due to multiple handling.    Milk is pasteurized and packaged for retail sale at or near the origin and then shipped in refrigerated containers to the destination. The result is higher cost and a loss of shelf life at retail due to the transit period.
The repeated pasteurization of the first option is also particularly undesirable because while most milk borne microorganisms are neutralized by pasteurization, their lipolytic and proteolytic enzymes can survive and result in undesirable lipolysis and proteolysis.
The major strategy to extend shelf life of unpasteurized perishable organic liquids has been to provide rapid refrigeration. For instance, decreasing the storage temperature from 6° C. to 2° C. increases the time for the psychrotrophic count to reach 106 cfu (colony forming units)/ml from 2.9 to 5 days (Griffith, 1987).
Several authors have reported on the use of unpressurized carbon dioxide as an anti-microbial agent in foods including dairy products. The concept of using CO2 to inhibit the growth of unwanted microorganisms in dairy products stems from the technology of modified atmosphere packaging. This method of shelf life extension has been adapted to fluid dairy products by directly injecting the inert gas (CO2) thereby enhancing its inhibitory effect. The direct post-pasteurization addition of carbon dioxide (DAC) to neutral and acidic pH products can be used to control contaminating organisms. DAC is widely used by cottage cheese processors in North America. Carbon dioxide has also been shown to extend the shelf life of yogurt, to improve the keeping quality of raw milk, and to extend the yields of cheese subsequently prepared from such milk. However, under specific combinations of pressure and temperature, CO2 effectively precipitates the proteins from milk. For example, at 38° C. and pressures above 5514 kiloPascals (kPa), or about 800 psi, complete precipitation of the casein proteins that give milk its distinctive white color results. CO2 pressure treatments applied at a pressure of only 294 kPa (about 43 psi) at 20° C. may result in casein aggregation. Accordingly, pressurization has been avoided due to potential deleterious effects upon the treated liquids. In addition, and not unrelatedly, there is an absence of suitable pressure vessels for pressurized bulk storage and transport of organic liquids. The studies utilizing CO2 pressure treatments have been principally directed to pathogen reduction treatments with high CO2 pressures as an alternative to thermal pasteurization. Lower CO2 pressures have not been previously utilized as conditions of storage and transportation to reduce microbial growth.